Dehydrating caustic alkali solutions



Patented Feb. 28, 1933 UNITED STATES PATENT. o1=1=1cE Y CLARENCE E.MILLER, OF CANTON, NORTH CAROLINA, ASSIGNOR TO THE CHAMPION m3 Drawing.

FIBRE COMPANY, OF CANTON, NORTH CAROLINA,

A CORPORATION OF OHIO IDEHYDRATING CAUSTIC ALKALI SOLUTIONS carbonmonoxide or other constituents of the alkalimetal hydroxides andparticularly so 7 gaseous combustion products from the commonly-usedsolid, liquid or gaseous fuels. The invention is especially applicablein the evaporation or dehydration of solutions of in those cases inwhich such hydroxides are preparedv by electrolysis of an aqueoussolution of analkali metal salt with simultaneous evolution of hydrogenas a by-product.

It heretofore has been proposed to dehydrate electrolytically producedaqueous solutions of alkali metal hydroxide, e. g., sodium hydroxide, byfirst subjecting the cell liquor to evaporation in multiple effectevaporators, with removal of salt as the evaporation progresses, thesaid treatment being continued until the intermediate product represents about a 50% solution of the hydroxide, and then transferring theso-obtained 50% solution to a large open evaporating pot hea ed by acoal fire, oil blast, or the like, in which pot the solution is heatedat atmospheric pressure either to dehydration or to a concentrationhigher than the gaseous or vaporous products of evaporation beingconducted by means of a hood from the pot to apoint removed therefrom.The efiiciency of heat transfer in this commonly employed method isknown to be very low and, inasmuchas the surface for evaporation islimited, the method is a very slow one. Furthermore, contamination ofthe evaporated proclucts with the products of combustion of thefuelemployed, with formation of carbonates of the metals, is practicallyunavoidable. So expensive and inefiicient is this commonly employedmethod that investigators have proposed further evaporating partiallyevaporated, e. g., 50%, solutions of alkali metal hydroxides in closedjacketed vessels through the jackets of which highly heated vapors areconducted, the vaporous evaporation products being removed from thevessels by any appropriate means such as, for instance, a vapor systemoperated at subatmospheric pressure. While such method may result inApplication filed August 19, 1031. Serial No. 558,187.

relatively very speedy dehydration of the solutions and an increasedefliciency, the complicated special apparatus therefor makes the methodexpensive. An object of the present invention is the provision of asimple yet highly economical process for evaporating or dehydratingsolutions of metallic hydroxides, which method may be practiced with theuse of comparatively simple and inexpensive evaporation equipment. Amongother objects thereof is the provision of a process foreva'poratingordehydrating solutions of metallic hydroxides which is not only veryrapid but also effects avoidance of contamination of the evaporatedproduct. w r p These, and other, objects of invention are found to beattained by exposing the alkali metal hydroxide solution to beevaporated, preferably in thin layers or in finely'divided form or inany other form providing re1atively large layers of the solution,directly in contact with burning hydrogen. That ,is to say, theinvention resides in the evaporation of aqueous solutions of alkalimetal hydroxides or their equivalents by direct contact combustion ofhydrogen. High evaporation efliciency is attained thereby and, since theproducts of the combustion are steam and nitrogen, there is therebyeliminated any possibility of contaminating the evaporated product withthe combustion products. Obviously, this process finds particularapplicability in the evaporation ordehydration of alkali metal hydroxidesolutions obtained by the electrolysis of solutions of alkali metalsalts with simultaneous evolution of hydr0- gen, in that by the. processforming the basis of the present invention there is provided aneconomically important outlet for the by product hydrogen. a

In this connection, it is to be noted that in the electrolyticproduction of, say, twenty (20) tons of caustic soda there is producedby the electrolysis approximately one thousand (1000) pounds of hydrogenWhich,under standard conditions, is equivalent to one hundred eightythousand (180,000) cubic feet of hydrogen. Assuming the usual heat valueof 280 'B. t.'u. per cubic foot for the hydrogen, there is availablefrom this amount of hydrogen approximately fifty million four hundredthousand (50,400,000) B. t. u.s of heat by the complete combustion ofthe said hydrogen. The 50% caustic soda solution which may be obtainedby the evaporation of the cell liquor from the aforesaid electrolysis(the salt being removed during evaporation to 50% strength) containsapproximately twenty tons of water which, by standard calculation,requires ap-, proximately forty million (40,000,000) B. t. u.s of heatfor its evaporation. It will be seen, therefore, that upon a theoreticalbasis the by-product hydrogen is more than sufficient to supply the heatnecessary for the complete evaporation of the water present in thesolution obtained from the cell liquor.'

The process forming the basis ofthepresent invention makes possible, anddesirable, the

utilization of the heat contained in the products of combustion of thehydrogen in anysuitable heat exchanger, such, for instance, as

. a tubular boiler for the generation of steam for power and otherpurposes, the nitrogen in the gas either being allowed to escape orbeing employed for some useful purpose.

While it is not the intention of this application to restrict theworking of the abovedescribed process to the use of any particu larapparatus, the method being adapted to be carried out in any one of anumber of types of existing apparatus used 1n chemical industries, ithas been found to be preferable to inject the partially concentratedsolution (e. g., 50% solution) of the alkali metal hydroxide in finelydivided form, e. g., as a spray, into a closed space into which thehydrogen and a gaseous supporter of its combustion are caused to passand there burn, which closed space is provided with suitable means forremoving the dehydrated product and means for conducting away thegaseous products of combustion and the water evaporated fromthe'solution, the latter means preferably including a heat exchanger.The heat of combustion of the hydrogen evaporates the water from thestream of finely dixided partially concentrated solution, the

resulting dehydrated, or substantially dehydrated, caustic soda droppingto the bottom of the closed space and being removed therefrom in anysuitable manner as occasion requires, and the combustion products andvaporized solvent passing out of the space. Such apparatus is alreadyknown: see Liddell, Handbook of Chemical Engineering (1922), pages 389and 390, and therefor forms no part of the present invention.

As will be apparent from the foregoing, while it is not essential to theworking of the broad invention to use as the hydrogen fuel for thedehydration the byproduct hydrogen from electrolytic cells, and while itis not essential that any of the heat contained in the combustion gasesbe saved, the best economic working of the invention includes both ofthese features.

It is to be understood that the present invention is not restricted tocomplete dehydration of the hydroxide, or to the use of a hydroxidesolution of any specific strength or concentration, these beingvariables determined in part by the requirements in any specificpractical operation of the'process. Furthermore, it is not essentialthat the contact between the partially concentrated solution and theburning hydrogen take place in exactly the manner indicated: itmay' beeffected by passing the burning hydrogen and/or its hot gaseouscombustion products counter currently in direct contact with a movinglayer or film of the solution, or in some other suitable manner. Nor isitessenlution either to the flame itself or to the hot gaseouscombustion products thereof, these being equivalents from the standpointof the present invention. i

hat I claim is:

1. Process of evaporating a solution of an alkali metal hydroxide whichcomprises exposing the latter directly in contact with burning hydrogen.p 5

2. Process of evaporating a solution of alkali metal hydroxide whichcomprises exposing large surfaces of the latter directly in contact withburning hydrogen. I

3. Process of evaporating a solution of an alkali metal hydroxide whichcomprises maintaining combustion of hydrogen in an atmosphere of agaseous supporter of combustion in direct contact with a large surfaceof the said solution.

4. Process of evaporating a solution of an alkali metal hydroxide whichcomprises directly contacting a finely divided stream of the solutionwith burning hydrogen.

5. Process as defined in claim 1, in which the solution so exposed is apartially concentrated aqueous solution of the hydroxide.

6. Process as defined in claim 1, in which a previously partiallyconcentrated solution of the hydroxide is so dehydrated.

7 Process as defined in claim 1, in which the hydroxide is caustic soda.

In testimony whereof, I affix my signature.

' CLARENCE E. MILLER.

